The invention relates to a tire pressure sensor for motor vehicles and which is to be secured at the wheel rim circumference and is actuatable by the tire pressure. The sensor comprises a reference pressure chamber and an electrically conducting diaphragm which separates the reference chamber from the tire and cooperates with a contact pin extending through the center of the base of the reference pressure chamber. In such a known pressure sensor (EP-A- 0 157 205), two pressure measuring elememts with different threshold values for monitoring the tire pressure are fastened next to one another in the circumferential direction at the wheel rim. In vehicles for which different tire pressures are provided at the front wheels and the rear wheels, the different threshold values make it possible to exchange the wheels between the front and rear axle at any time without the pressure measuring elements also having to be exchanged simultaneously. The two threshold values are adjusted to the different tire pressures, respectively.
An excess pressure in the tire, at which the driving comfort and possibly also the driving behavior of the vehicle is impaired due to the reduced contact of the tire with the road, can also be determined and indicated, if necessary, by such a sensor when tires with a lower required tire pressure are used. However, such monitoring of the excess tire pressure is not possible in the tires filled at higher pressure. Moreover, with the sensors of this type, the correct functioning of the tire pressure sensor can hardly be monitored after installation at the rim. Thus, for example, it is possible that the diaphragm of the tire pressure sensor or a protective cap thereof is pressed in when exchanging the tires, and the switching function of the diaphragm is accordingly blocked. In such cases, a sufficient air pressure in the tire is also indicated during the tire pressure monitoring while the air pressure lies below the threshold value of the tire pressure sensor.